The present invention is directed to a method for measuring the condition of a steel structure. Furthermore, it comprises a device for carrying out such measurements of condition of a steel structure. The method can be adapted for detection of mechanical stress, fatigue and metal loss e.g caused by corrosion.
More particularly the invention concerns a method for measuring the condition of a steel structure, whereby a pulse-shaped electrical current is applied to the steel through a pair of contact points, generating potential drop curves between another pair of contact points measured by repeated sampling. The steel subjected to the measurements can both be magnetized and demagnetized by means of arrangements known per se and being employed for magnetization of the steel to selected points at the initial, the anhysteretic and hysteretic magnetization curves. Comparison of the potential drop curves at different magnetization makes it possible to determine the condition of the steel or change of condition. Change of some types of conditions may be measured without using magnetization means, in such cases the method also can be applied for monitoring of all types of electrically conductive materials.
Large steel structures are often subjected to varying loads causing deterioration. For safety reasons it is important to inspect and monitor such structures in order to discover conditions which can develop into serious defects, among these conditions being mechanical stresses, corrosion and erosion.
Mechanical stress can arise unintentionally both during construction, (for example in or adjacent to weld seams) and during operation because of uncontrollable conditions.
For measuring relative mechanical stresses strain gauges or other means are often employed for detecting small dimensional changes in the steel. By using of such methods changes are measured from the instant the arrangement is installed with respect to absolute mechanical stresses the possibilities are more limited. It is known, however, that measurement and analysis of Barkhausen noise can provide information about mechanical stresses. (Rautioaho, R. H., and Karjalinen, L. P.: “Application of Barkhausen noise measurements to residual stress analysis in structural steels”. Proc. Scandinavian Symposium in Materials Science, 20-21 Jun. 1983, OuIu Finland, pp 179-185.)
Moreover it is to be noted that mechanical stresses lead to changes in magnetic properties and use of this phenomenon has been attempted as a basis for measuring mechanical stresses in steel. (Lugg M. C: The effect of stress on the ACFM technique”. Controller C HMSO, London 1987.).
A method based on measuring the stationary voltage or potential drop from the excitation of a pulse shaped current is known from the Norwegian patent no. 150136.
The voltage drop is measured between a number of contact points both when the structure is in a preferably known initial condition and during operation. The voltage drops measured are compared with each other and form the basis for detecting changes in the condition of the structure caused by cracks and material loss.
The current applied is pulse shaped. Upon each current step the voltage drop measured will follow a transient curve because of the skin effect, which in the above patent is referred to as disturbances, and the method assumes that the transient portion of the voltage drop has decayed before measurement takes place. Thus, according to the patent a prejudice exists against the consideration of the transient portion of the voltage drop curve to provide information on possible defects or changes in the structure. The disadvantage is that for large pipes with large wall thickness the transient time is long and for practical applications and for other reasons measurements are made before the transient has decayed. These measurements are thus influenced by changes in the ferromagnetic properties caused by changes in the structures' stress or pipes' internal pressure, which are regarded as disturbances in the measurement of corrosion or cracks.
Norwegian patent specification 172868 describes a method to measure mechanical stress and fatigue in steel. This patent mainly describes a method based on measuring the transient voltage response from a pulse shaped excitation current. Two such transient voltage drops are measured between a number of contact points for different conditions of the structure. The measured transient voltages are compared and make the basis for computation of the mechanical stress and degree of fatigue. However, this patent does not describe how the method responds to fatigue. Furthermore, these measurements do not consider the information inherent in the stationary part of the voltage response, which includes significant information about the fatigue.